Internal combustion engines may be equipped with various exhaust gas aftertreatment systems for reducing pollutant emissions. In spark-ignition engines catalytic reactors are used, which ensure oxidation of HC and CO even at low temperatures by using catalytic materials which increase the speed of certain reactions. If nitrogen oxides NOx are also to be reduced in addition, this can be achieved through the use of a three-way catalytic converter, which to do this, however, stoichiometric operation (λ≈1) of the spark-ignition engine is used running within tight limits. In this case the nitrogen oxides NOx are reduced by means of the unoxidized exhaust gas constituents present, that is to say by the carbon monoxides CO and the unburned hydrocarbons HC, these exhaust gas constituents being simultaneously oxidized.
Usually and in the context of the present disclosure the air ratio λ is defined as the ratio of the air mass mair,act actually admitted to at least the one cylinder of the internal combustion engine to the stoichiometric air mass mair,stoich, which would be required in order to just fully oxidize the fuel mass mfuel admitted to at least the one cylinder (stoichiometric operation of the internal combustion engine λ=1). Here λ=mair,act/mair,stoich and with the stoichiometric air demand Lstoich, which is defined as Lstoich=mair,stoich/mfuel, the air ratio λ=mair,act/mfuel*(1/Lstoich).
In the case of internal combustion engines which are operated with an excess of air, for example diesel engines or direct-injection spark-ignition engines, but also in lean-burn spark-ignition engines, the nitrogen oxides NOx present in the exhaust gas cannot be reduced owing to the absence of reducing agents.
Consequently an exhaust gas aftertreatment system, for example a storage catalytic converter, which is also referred to as a lean NOx trap (LNT), may be provided for reducing the nitrogen oxides. Here the nitrogen oxides are at first absorbed, that is to say collected and stored, during lean-burn operation (λ>1) of the internal combustion engine, before being reduced during a regeneration phase, for example by means of a sub-stoichiometric operation (λ<1) of the internal combustion engine with oxygen deficiency, the unburned hydrocarbons HC and the carbon monoxide CO present in the exhaust gas serving as reducing agents.
Exhaust gas recirculation (EGR) and throttling of the charge air in the intake system afford further internal possibilities within the engine for enriching the exhaust gas with reducing agents, in particular with unburned hydrocarbons. Both measures reduce the charge air mass or fresh air mass fed through the internal combustion engine and thereby reduce the air ratio λ. Enrichment ensues due to reduction of the air mass supplied.
An enrichment of the exhaust gas with unburned hydrocarbons, also termed HC enrichment, can also be achieved by means of post-injection of additional fuel into at least one cylinder of the internal combustion engine. Here the post-injected fuel is not intended to be ignited in the combustion chamber by the main combustion still underway, or by the combustion gas temperatures, which are still high even after completion of the main combustion, but is to be introduced, unburned, into the exhaust gas discharge system upstream of the catalytic converter during the charge cycle.
Internal combustion engines which make use of post-injection are naturally prone to a dilution or contamination of the oil by unburned hydrocarbons. Depending on the quantity of post-injected fuel and the point of injection, a greater or lesser proportion of the post-injected fuel gets onto the inside wall of the cylinder, where it mixes with the adhering oil film and thus contributes to the oil dilution. Furthermore, the use of additional fuel as reducing agent is bound to increase the overall fuel consumption of the internal combustion engine.
With regard to the sub-stoichiometric operation (λ<1) of the internal combustion engine, that is to say the enrichment, the initiation and maintenance of sub-stoichiometric operation can sometimes only be accomplished subject to restrictions, if at all. The reasons for this are diverse and vary as a function of the instantaneous load at which the internal combustion engine is being operated.
At low loads a stable combustion cannot be ensured when running with a rich mixture, particularly using compression ignition. Misfiring or incomplete combustion of the mixture may occur. The consequence is an undesirably high pollutant emission, particularly of unburned hydrocarbons HC. In the middle load range a load fluctuation often occurs. The transient operating conditions make it more difficult to maintain a constant air ratio and in some instances render any enrichment impossible. In the higher, high or top load range sub-stoichiometric operation is usually governed by the maximum admissible exhaust gas temperature, the exhaust gas temperature often being limited by components provided in the exhaust gas discharge system and their capacity to withstand thermal loads, for example by the turbine of an exhaust turbocharger, an exhaust gas aftertreatment system, or the exhaust gas recirculation system. It has to be borne in mind in this context that typically the exhaust gas temperature rises with any enrichment.
Internal measures within the engine can be dispensed with if the reducing agent is introduced directly into the exhaust gas discharge system, for example through the injection of additional fuel upstream of the LNT.
In the method and the internal combustion engine which form the subject matter of the present disclosure, the exhaust gas is enriched by introducing reducing agent into the exhaust gas discharge system upstream of at least the one exhaust gas aftertreatment system. Further measures for enriching the exhaust gas, particularly internal measures within the engine, may also be provided, however.
During the regeneration phase the nitrogen oxides (NOx) are released and substantially converted into nitrogen dioxide (N2), carbon dioxide (CO2) and water (H2O). The temperature of the storage catalytic converter should preferably lie within a temperature window between 200° C. and 450° C., so that on the one hand a rapid reduction is ensured and on the other hand no desorption takes place without conversion of the rereleased nitrogen oxides NOx, something which can be triggered by too high temperatures.
One problem in using a storage catalytic converter arises from the sulfur contained in the exhaust gas, which is likewise absorbed and has to be regularly removed as part of a so-called desulfurization. For this purpose the storage catalytic converter is heated to high temperatures, usually between 600° C. and 700° C., and supplied with a reducing agent, for example unburned hydrocarbons. The high temperatures required for desulfurization can damage the storage catalytic converter, contribute to thermal ageing of the catalytic converter and significantly reduce the desired conversion of nitrogen oxides towards the end of its service life.
Although the problems resulting from enrichment by means of internal measures within the engine can be eliminated or moderated through the introduction of fuel, serving as reducing agent, directly into the exhaust gas discharge system, even this concept of exhaust gas enrichment is of only limited use, since the introduction of additional fuel into the hot exhaust gas and the accompanying exothermic reaction increase the already high temperature of the exhaust gas of an internal combustion engine in operation, possibly to values in excess of admissible exhaust gas temperatures, so that a thermal overload can occur. Reference is made to the comments already made above.
Instead of a storage catalytic converter or in addition to a storage catalytic converter, a selective catalytic converter, which is also referred to as an SCR catalytic converter, can be provided for reducing the nitrogen oxides. What has been stated in connection with the storage catalytic converter applies with regard to the supply of reducing agent. Besides unburned hydrocarbons, ammonia NH3 and urea are used as reducing agents in order to reduce the nitrogen oxides selectively. The last-mentioned reducing agents are purposely introduced into the exhaust gas, that is to say directly into the exhaust gas discharge system.
The technical correlations described above illustrate that improved procedures for exhaust gas enrichment are needed, so that exhaust gas aftertreatment systems for reducing nitrogen oxides can be optimally supplied with reducing agent, in order to regenerate or desulfurize a storage catalytic converter, for example, or to supply a selective catalytic converter with unburned hydrocarbons or ammonia.
In the light of the comments above, the inventors herein provide an approach for regenerating an exhaust aftertreatment device that avoids exposure of the device to excessive temperatures while alleviating the combustion issues that may arise during rich operation. Accordingly, a method for enriching the exhaust gas of an internal combustion engine with a reducing agent is provided. The internal combustion engine comprises at least one cylinder having an intake system for admission of charge air, an exhaust gas discharge system for discharging exhaust gases, and at least one exhaust gas aftertreatment system for reducing nitrogen oxides arranged in the exhaust gas discharge system and which is periodically provided with a supply of reducing agent. The method comprises enriching the exhaust gas with the reducing agent in the exhaust gas discharge system upstream of at least the one exhaust gas aftertreatment system when a fuel supply of the at least one cylinder of the internal combustion engine is deactivated due to an absence of load demand.
According to the disclosure enrichment of the exhaust gas with reducing agent is not undertaken when the internal combustion engine is operating under load, but rather due to the absence of load demand when, with the fuel supply deactivated, no combustion is initiated in the cylinders of the internal combustion engine for lack of fuel, and the internal combustion engine functioning as a piston machine delivers charge air whilst consuming power. Enrichment of the exhaust gas is thereby performed in an operating mode of the internal combustion engine, which according to the state of the art is deemed unsuitable for an enrichment of the exhaust gas, since on the one hand fuel as reducing agent is entirely lacking due to deactivation of the fuel supply, and on the other hand exothermic heat resulting from the combustion, for raising the relevant temperatures, particularly of the exhaust gas and the exhaust gas aftertreatment system, is lacking due to the absence of fuel combustion.
However, the residual heat of the components, particularly at least the one exhaust gas aftertreatment system, proves to be sufficient. It even proves advantageous, according to the disclosure, for the reducing agent to be introduced into the exhaust gas discharge system when the internal combustion engine is not operating, since in this way the risk of thermal overload due to excessively high temperatures is reduced or eliminated altogether.
Here the reducing agent is not introduced into the hot exhaust gas discharged from the cylinders, but rather into the charge air fed through the internal combustion engine and into the exhaust gas discharge system, so that strictly speaking the method according to the disclosure involves charge air enrichment and not enrichment of the exhaust gas. Nonetheless, the term exhaust gas enrichment known from the state of the art is retained.
Both the disadvantages resulting from the provision of reducing agent by mean of internal measures within the engine, and the thermal overheating of components provided in the exhaust gas discharge system, particularly due to the direct introduction of reducing agent into the exhaust gas, are eliminated.
Moreover, using the method according to the disclosure in the normal operation of the internal combustion engine imposes fewer restrictions, since reducing agent for enriching the exhaust gas needs to be provided less frequently, if at all, and particularly under load demand no internal measures within the engine for HC-enrichment need to be implemented. It also has to be borne in mind here that enrichment during normal operation may be avoided, since the introduction of excess fuel is to be regarded as detrimental from various energy standpoints, particularly with regard to the efficiency of the internal combustion engine, but also with regard to the pollutant emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.